People have mentioned Boba Fett, Han Solo, Yoda etc but why can't there be a spinoff based on a brand new character

There very well could be.

People are only talking about Original and Prequel trilogy characters because until TFA was released that was all we knew about. The speculation was already a year or more old so it is going to take a while for people to consider TFA characters. Plus Lucasfilm need to be at least in production on Episode IX before adding a spinoff to the Sequel era - they need to have the trilogy set down in stone.

This is actual the very reason I came to this section - to find out how many people think that the the third spin off will be set in the sequel era. Or be set in the years prior to TFA.

Poe would be ideal, because he has been involved in the Resistance and he has been in the New Republic. It would allow us to experience and see more of the Resistance up close and experience the New Republic prior to TFA.

__________________"When the snows fall, when the white winds blow, the lone wolf dies, but the pack survives."